1. Field of the Invention
This invention relates to a control device for an electric train which is driven with several carriages coupled together.
2. Discussion of Background
In most railway carriages, power for the various kinds of control devices, air-conditioning, lighting equipment, etc. mounted in the carriages is obtained from a power converter connected to a current collector (referred to below as a pantograph). This power converter convert the high voltage of the overhead line to a voltage suitable for the various devices and thus supplies these devices with power. An inverter may be used, for example. When there is a momentary interruption of the input power from the overhead line, owing to loss of contact with the overhead line due to vibration of the pantograph or the passage of the carriage thoough a dead section, a protective device stops this inverter from operating. The inverter is restarted later, when prescribed conditions have been fulfilled. When the inverter stops and restarts, devices such as (for example) air-conditioning devices constituting the load cease operating temporarily and are later restarted successively. The fact that the air-conditioning and lighting equipment cease operating every time power input is interrupted because of loss of contact between the pantograph and the overhead wire or passage through a dead section results in a deterioration of the service to passengers.
The conventional approach for dealing with this momentary interruption of power, when several carriages are coupled together, is to connect all the pantographs in parallel by means of a through-lead. With such a through-lead connection, even if one pantograph loses contact with the overhead wire or passes through a dead section, power is obtained from the other pantographs, which are still in contact with the overhead line. Another approach is to enlarge the capacity of the filter condenser provided on the DC input side of the inverter, so that during a momentary power failure the inverter is driven by discharge from this filter condenser.
However, the through-lead which is provided between carriages to connect the pantographs in parallel connects them in parallel directly, and is therefore a high-voltage line. This gives rise to a safety problem, and various protective devices have to be provided to ensure that safety is maintained.
On the other hand, the fact that the filter condenser is mounted on the carriage imposes restrictions on its weight and size, and the extent to which its electrostatic capacity can be increased is limited.
Consider, for example, the relation between the filter condenser voltage and the period of loss of contact between the pantograph and the overhead line, as shown in FIG. 4. If the voltage of the filter condenser when charged up to the voltage of the overhead line is taken as V1, and the filter condenser voltage at T seconds after loss of contact between the pantograph and the overhead line as V2, we have the following approximate equation. EQU 1/2C (V1.sup.2 -V2.sup.2)=130.times.10.sup.3 .times.T (1)
Now if V1=1500V (overhead line voltage), V2=900V (value set for low voltage protection), and C=1500 .mu.F, then T is approximately equal to 8 ms. This means that at approximately 8 ms after the pantograph has lost contact with the overhead line, the voltage of the filter condenser, which was charged up to 1500 V, drops to 900 V. When the voltage of the filter condenser falls as far as this figure of 900V, the value set for low voltage protection, a protective device stops the operation of the inverter. In practice, since th period during which the pantograph is out of contact with the overhead line often exceeds 10 ms, it is difficult to achieve a continued supply of power by means of discharge from the filter condenser when the main supply is interrupted.